Resonators are commonly used in electronics for providing a signal with accurate and stable frequency. The resonators are mostly made using quartz crystals, which have a good accuracy and temperature stability of frequency. However, the production process for producing crystal resonators is different from the process of producing most other electrical circuits, which are mainly produced of silicon. Therefore, the quartz crystal resonators are usually separate components, whereby separate phases are required in the production process of electronic devices.
The quartz crystal components also tend to be large in size. It would be desirable to provide MEMS resonators made of silicon or other semiconductor materials in order to facilitate integration with e.g. silicon based components.
One problem associated with silicon based resonators relates to their actuation. In electrostatic actuation electrodes are placed on one or more sides of the resonator body in such a way that a narrow gap is formed in between the resonator body and the electrode. A voltage between the resonator and electrodes results in an electrostatic force, which can be used for driving square-extensional or Lamé resonance. For example, Mattila et al, “Silicon Micromechanical Resonators for RF-Applications”, Physica Scripta. Vol. T114, 181-183, 2004, show an electrostatically actuated silicon resonator exhibiting square-extensional mode. However, to obtain strong enough electromechanical coupling, electrostatic actuation requires in general large (>20 V) bias voltages and narrow (<200 nm) gaps between a transducer element and the resonator. The bias and gap constraints are considerable disadvantages as concerns IC design and MEMS processing. In particular, the bias constraint is a complication for oscillator drive IC design, as low-cost processes are not compatible with voltages less than ˜5V and the on-IC DC voltage generation is power consuming. The gap constraint is a process complication, because typical commercial MEMS processes are only capable of >=2 μm gaps. Narrow gaps are also an ESD risk, reducing device reliability.
It is also known to use piezoelectric actuation with a piezoelectric film grown on the resonator structure, but this approach is suitable only for exciting certain resonance modes. For example, the Lamé mode is problematic for piezoelectric actuation methods in single-crystal resonators such as silicon plate resonators. Previously, the Lamé mode has been piezoelectrically successfully produced only to quartz or special ceramic crystal structures such as 155° rotated Y-cut LiNbO3 plates (e.g. Nakamura, K. et al, “Lame-mode piezoelectric resonators and transformers using LiNbO3 crystals” Ultrasonics Symposium, 1995. Proceedings., 1995 IEEE, 7-10 Nov. 1995, vol. 2, 999-1002).
Another problem associated with silicon based resonators is that they have a high temperature drift of the resonance frequency. The drift is mainly due to the temperature dependence of the Young modulus of silicon, which causes a temperature coefficient of frequency (TCF) approx. −30 ppm/C. This causes the resonance frequency to fluctuate due to changes in ambient temperature. It is possible to compensate the temperature dependence with a temperature sensor and related electronic control circuitry, but it has not been possible to provide a resonator with sufficiently low temperature drift with low cost technology which would be suitable for mass production applications and would compete with quartz quality. Also, the use of a temperature compensation circuit increases the consumption of energy, which is a significant disadvantage especially in battery operated devices. Further, the compensation circuit tends to increase electric noise in the resonator circuit. It is also possible to stabilize the temperature of the resonator with temperature isolation and controlled warming/cooling of the resonator. However, this solution also increases the energy consumption of the device, and makes the device complicated to produce. The temperature compensation circuits are also slow in controlling, and cannot therefore compensate fast or large changes in ambient temperature sufficiently well.
J. S. Wang et al, “Sputtered C-Axis Inclined Piezoelectric Films and Shear Wave Resonators”, Presented at the 37th Frequency Control Symp., Philadelphia, 1-3 Jun. 1983, 1983, 1-3, present that the p+-doped silicon appears to have positive temperature coefficient and in combination with ZnO and AlN films such silicon can be used for manufacturing inclined-angle shear mode resonators with an overall temperature coefficient near zero.
Lately it has been shown by A. K. Samarao et al, “Passive TCF Compensation in High Q Silicon Micromechanical Resonators,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2010), 2010, pp. 116-119, that heavy p-doping of silicon dramatically reduces the TCF of a bulk acoustic wave resonator with enhanced shear wave contribution. However, the abovementioned problems relating to actuation of the resonator remain.
US 2010/0127596 discloses a MEMS resonator which includes a boron-doped resonator region in order to reduce the TCF. The resonator may comprise a piezoelectric layer on top of the resonator and input/output electrodes on top of the piezoelectric layer. Such structure is not capable of producing a Lamé mode in a plate-shaped resonator.
It has also been suggested to use composite structures in resonators where there are layers with opposite temperature coefficients. Document U.S. Pat. No. 4,719,383 discloses a shear wave resonator structure wherein a resonating beam has an obliquely grown piezoelectric layer and a p+ doped silicon layer. While the piezoelectric layer has a negative temperature coefficient, a heavily p+ doped silicon layer has a positive temperature coefficient. The thicknesses of the piezoelectric and doped silicon layers are made such that the total temperature coefficient of the resonator is near to zero.
There are certain disadvantages related with resonators of such composite structure as well. Firstly, the p+ doping of U.S. Pat. No. 4,719,383 is made by diffusion via the material surface. Diffusion is typically a slow process, and therefore the doped layer cannot be very thick. Increasing the thickness of the silicon layer would also cause the coupling of the actuation to be worse. As a result, since the resonance frequency is a function of the total thickness of the resonator structure it is only possible to provide resonators with high frequencies. The patent document mentions suitable frequencies above 300 MHz. However, there are numerous applications where lower resonance frequencies are required, for example in the range of 1-100 MHz, in particular 10-100 MHz, to be used for example as a reference frequency. The solution of U.S. Pat. No. 4,719,383 is not feasible for such lower resonance frequencies.
Another problem relating to the composite structure of U.S. Pat. No. 4,719,383 relates to the accuracy of the resonance frequency. In a thickness oriented shear wave resonator the resonance frequency is determined by the thickness of the resonator structure, and therefore an accurate resonance frequency requires achieving an accurate thickness of the resonator structure. However, it appears very difficult to achieve sufficient accuracy of the thickness, and therefore it is difficult to achieve the required accuracy of resonance frequency. In mass production, the deviation of resonance frequencies of such resonators tend to be high, and thus the yield of resonators which fulfill the required specifications tends to become low unless improved by local correction by e.g. ion beam etching, which, however, increases process complexity and cost substantially. A further problem which relates to the prior art MEMS resonators based on beam vibration is the fact that the small-size resonator beam has a small oscillating mass, and therefore the resonator is able to store only a small amount of oscillation energy. This in turn causes a low signal-to-noise ratio of the resonator and thus instability of the output signal frequency.